The Online documentation for IPv6 is not dealing with the case of Debian 8, Ubuntu, or any distribution using systemd. Systemd replaces upstart, so the procedure has to be extended to add systemd service for startup, replacing the previoux behavior that was using init scripts.

Ensure that you are IPv6-proof

It should be the case as Debian 8 is shipping a kernel with native IPv6, but just to be sure :

In /etc/modprobe.d/local.conf :

options ipv6 disable=0

In /etc/modules :

ipv6

You may have to reboot in order to apply the changes.

Set up your IPv6 prefix and get your DUID on Online console

Make a /64 and do not use your /48 or your /56 directly, as you may regret it. You can have only one /48 by account, one /56 by server, and one /64 by IP failover (the /48 is divided to make the /56 and so on).

Configure your network interface

It may not be mandatory as Dibbler will reconfigure your interface, but you have to ensure that you accept router advertisements. Add to /etc/network/interfaces :

Notes about Proxmox and forwarding

On Proxmox, you are working on the bridge interface, it should be vmbr0 instead of eth0.

If you enabled forwarding on this interface (to give your VM an access to IPv6 network), you have to force the accept_ra to 2, while the default value of 1 wil make your Debian to ignore router advertisements when forwarding is enabled ! Read more. Add to sysctl.conf :

net.ipv6.conf.vmbr0.accept_ra = 2

Also do not set the IPv6 interface of your VM to vmbr0, as you can break your network access. Read more.

Addendum : just in case, don’t use resolvconf with IPv6 DNS pushing and if possible keep IPv4 DNS, it would be such a pity if the IPv6 networking crashes while your DNS servers are set to IPv6 addresses (yeah, it happened to to me)…